Okay, if the light blinks and eventually quits, and is sometimes accompanied by a chirping or clicking noise from inside the dash behind the radio, that's a bad air bag controller. It's the light blue box behind the radio.
If like dreger, it blinks a set number of times and keeps repeating it over and over again. The controller may be fine, but it's trying to tell you something.
I always thought code 6 was a bad steering wheel clock spring? It's probably the most common air bag trouble code.
Here's some helpful information regarding Ford air bags
Mustang Air Bag Diagnostic Codes